Deuteration-induced phase transition in ammonium hexachloroplumbate

The heat capacities of (NH4)2PbCl6 and (ND4)2PbCl6 were measured at temperatures between 13 K and 300 K. A gradual phase transition was found at 77.8±0.3 K for (NH4)2PbCl6 with an associated transition entropy of 3.0 J K−1 mol−1. The transition temperature shifted to 81.0±0.3 K in (ND4)2PbCl6. A sharp first order transition occurred at 38.40±0.03 K for (ND4)2PbCl6 accompanied by a transition entropy of 7.5 J K−1 mol−1. There was no corresponding phase change in (NH4)2PbCl6. The mechanism of the deuteration-induced transitions in the present compound and similar transitions in (ND4)2TeCl6, (ND4)2PtCl6, (ND4)2PdCl6, and (ND4)2SeCl6 is discussed. Tunneling energies involved in the rotational motion of the ND4+ ion in the three-minimum potential were derived from the experimental entropy data. They correlate smoothly with the metal-chlorine bond distance for these five isomorphous compounds. Standard thermodynamic functions calculated from the heat capacity data are tabulated in the Appendix.